The present invention relates to Scophony raster output scanners (ROS) used for electronic image transmission and processing, such as laser printers and facsimile machines. A Scophony raster output scanner is a particular type of raster output scanner; and a flying spot scanner is also a particular type of raster output scanner.
A flying spot scanner (FSS) may be configured as an input scanner, as an output scanner, or as both. An input scanner scans an optical beam across an existing image or object, in order to form an "electronic image" which can be transmitted and/or processed. An output scanner scans a photoreceptor, in order to form an image on the photoreceptor. The present invention is concerned with output scanners, in particular with "raster output scanners" (ROS), which form an output image while scanning the photoreceptor in a predetermined pattern, or "raster".
In an ROS, a modulated optical beam is raster-scanned across a photoreceptor, in order to recreate, or form, an image. The raster pattern is usually a rectilinear scan, similar to that used on broadcast television, comprising a rapid horizontal line scan, and a relatively slow vertical motion. The rapid horizontal scan is usually provided by a rotating wheel having a number of flat mirror facets on its periphery. The slower vertical scan is usually provided by vertical motion of the photoreceptor. The architecture of a typical ROS is shown in FIG. 1, and in particular the architecture of a Scophony ROS is as described in "Scophony Spatial Light Modulator" in Optical Engineering (24:1) Jan/Feb 1985, pp. 93-100.
When a conventional flying spot scanner (FSS) is used as an ROS, as shown in FIG. 1, an optical modulator provides temporal modulation of the optical beam--i.e. the intensity of the entire optical beam is controlled. The modulator is typically an acousto-optical modulator (AOM). In this "conventional" approach, the optical beam impinging on the AOM must be no larger than the smallest modulatable area in the AOM as shown schematically in FIG. 2a and as detailed in FIG. 3a.
The "Scophony" raster output scanner (Scophony ROS), based on the AOM's spatial modulation capability, impinges a broad optical beam, shown as solid lines in FIG. 2b, on the AOM. The broad optical beam is "spatially" modulated; i.e. it is simultaneously modulated by several elements of the AOM, as shown in FIG. 2b and FIG. 3b. As shown in FIG. 2b, the AOM (object plane) is located at one image conjugate of the scanner optics; and the photoreceptor (image plane) is located at the other image conjugate.
FIG. 3 details the difference between the conventional FSS ROS and the Scophony ROS. FIG. 3a shows how the small laser spot of the conventional FSS ROS is modulated by one small area of the AOM. FIG. 3b shows how the broad laser beam of the Scophony type ROS is modulated by a large area of the AOM, an area which encompasses several signal pulses simultaneously.
The beam-modulating acoustic pulses in the AOM move at the speed of sound in the AOM material. Thus the image of the AOM's signals, formed on the optical detectors, would move rapidly across the photoreceptor. If not compensated for, this rapid motion would cause the image to be so blurred as to be useless. Since the beam is being rapidly scanned across the photodetector in a direction and speed that satisfies other system requirements, the Scophony optical system is designed so that the velocity of the image of the AOM's signals is equal to the beam scan velocity. Thus the image of the pulse train is stationary on the photoreceptor. As shown in FIG. 4c, when the mirror is rotating at the proper velocity, the pulses appear stationary, but the laser beam moves across the photoreceptor.
Because it can make maximum use of mirror facets which are tailored to the overall system bandwidth, the Scophony ROS using vestigial sideband modulation can provide higher resolution for a given size of mirror facet than it can when using the more conventional double sideband (DSB) modulation.
In addition to this advantage, the Scophony ROS using vestigial sideband modulation provides an increased depth of field, relative to an ROS using double sideband (DSB) modulation. Increased depth of field, in turn, makes system focus requirements less critical, thereby making it easier and less expensive to implement.
Finally, the Scophony ROS using vestigial sideband modulation provides an overall linear transfer function, because photoreceptors have a square law response to light amplitude, which is a requirement for a linear detection of vestigial signals. This means that, when used to transfer imagery characterized by shades of gray, rather than just black and white, the Scophony ROS can provide accurate rendition of imagery.
The problem of achieving all the performance improvements inherent in the Scophony raster output scanner (ROS) has presented a major challenge to designers in the ROS field. The development of a practical means of modulating an optical carrier with video information, while simultaneously providing a changing frequency correction to compensate for the motion of the ROS's mirror facet, would enable realization of all the inherent performance improvements. An invention which would enable achievement of this enhanced performance would satisfy a long felt need within the ROS community.